This invention relates to improvements in hydraulic positioning apparatus for precisely and rapidly positioning objects of the type likely to exert high impact or shock loads on the positioning apparatus. In particular, the invention relates to improvements in hydraulic positioning apparatus of the automatic type in which a controller responsive to the position of the apparatus automatically controls its movements so as to obtain precise positioning. The invention is especially applicable, although not limited, to sawmill setworks for precisely positioning logs preparatory to sawing thereof, and to the use of linear hydraulic piston and cylinder positioning assemblies in such setworks.
Precise automatic hydraulic positioning devices of both the rotary and linear (i.e. piston and cylinder) types have been known for many years. An example of an exceptionally precise automatic linear positioning assembly, of a type capable of positioning objects to within several thousandths of an inch, is described in U.S. Pat. No. 4,121,504 issued Oct. 24, 1978, the text of which is incorporated herein in its entirety by this reference. The rapid, yet precise positioning function preformed by devices of this type is made possible by the fine modulation of a servo valve which is controlled automatically by a controller responsive to the position of the movable element of the hydraulic positioning device. The actual position of the movable element is compared to a predetermined or desired position and the valve is modulated to achieve and retain such position. Moreover the automatic controller also controls acceleration, velocity and deceleration of the positioning device which is necessary for rapid as well as precise positioning. To accomplish all of this, the hydraulic components of the system, particularly the seals, servo valve modulating surfaces and internal moving surfaces of the hydraulic motor itself, must be in good condition so that undue leakage, friction or other obstacles do not hinder the precise control function.
Because of the foregoing requirements of such a hydraulic positioning system, difficulty has been encountered when attempting to apply such a system to the positioning of objects of the type expected to impose substantial impact loads on the system. Such impact loads can instantly increase hydraulic pressure to many times that for which the system was designed, leading to rapid deterioration or destruction of seals, valves and other components such that the system either becomes rapidly unreliable or requires an unreasonably high degree of continuous maintenance.
Previous methods of reducing the effect of impact loads on hydraulic systems have included interposing cushioning pneumatic bags or cylinders between the hydraulic motor and either its mount or the impact load. However these permit a nonrigid connection between the hydraulic device and the object to be positioned, and therefore are not compatible with the precision and quickness required of automatic positioning systems.
One commonly attempted method of attempting to overcome the problem of impact loading in automatic positioning systems is depicted in FIG. 7 of the aforementioned U.S. Pat. No. 4,121,504. This is the provision of a spring-operated pressure-relief valve which is intended to relieve overpressures which may occur on either side of the piston and cylinder assembly by exhausting overpressurized fluid to the sump. The problem with this arrangement is that the spring-operated pressure-relief valve is too slow to relieve instantaneously-applied impact pressure, especially when interposed between the servo control valve and the piston and cylinder assembly. The springs of such valves are heavy and operate over a substantial pressure range between full closure and full opening of the valve. In the time that it takes for the valve to open sufficiently to relieve impact pressure the damage from particularly high impact loads will already have occurred. What is needed is a system that is so fast-acting that the impact pressure is prevented from building up despite the instantaneous nature of the impact load.
A particularly appropriate potential application of automatic hydraulic positioning systems exists with respect to sawmill setworks wherein carriages will transversely-movable setting knees receive the position logs preparatory to sawing. The log is moved onto the carriage with substantial momentum and impacts with great force against the setting knees, which then position the log precisely for sawing so as to obtain optimum yield from the particular log. A plurality of setting knees are spaced along the carriage and, pursuant to modern requirements, the knees are movable independently of one another so as to achieve infinite angular adjustment of the log with respect to the saws (known as achieving "infinite taper"). It has been known to use linear hydraulic piston and cylinder assemblies to position the setting knees of such setworks, but these are of the much less efficient, nonautomatic manually-controlled type where the absence of automatic controls makes deterioration and imprecision of the hydraulic system more tolerable. Because of the extremely high impact loads imposed by the logs on such a system, however, no automatic linear hydraulic positioning systems having precise, rapid automatic positioning have previously been employed to position the setting knees. Instead, ballscrew drives have been employed where automatic positioning is desired. However a great disadvantage of using ballscrews has been the low setting speed of the knees. Also, to achieve infinite taper, each knee must have an individual ballscrew drive, which results in an extremely expensive system.